The present invention generally relates to equipment and processes for attaching components together by compressing one of the components around the other. This invention particularly relates to a system and process for crimping a fitting to the end of a conduit to consistently achieve a targeted crimp diameter for the fitting.
Crimping processes generally involve a set of tooling, such as a die set, that is forced closed around a fitting loosely assembled onto the end of a conduit, such as a hose or tube adapted to transport a fluid or protect electrical wiring or other hardware susceptible to damage. A representative hydraulic hose and fitting assembly 10 is depicted in FIG. 1 as including a hose 12 and fitting 14, with the end 16 of the hose 12 being received and trapped between an inner stem 18 and outer ferrule 20 of the fitting 14. The crimping action is the result of the die set (not shown), arranged around the circumference of the ferrule 20, being collapsed in a controlled manner onto the ferrule 20 to apply a crimping force radially inwardly toward the centerline of the hose 12, as indicated in FIG. 1. To meet reliability and manufacturing standards, the die set used to crimp the fitting 14 must sufficiently compress the fitting 14 to lock the fitting 14 onto the hose end 16. For this purpose, manufacturers typically designate a final crimp diameter “Dc” and allowable tolerance for a given type and size of fitting 14 and hose 12.
Crimping equipment often rely on a hydraulic cylinder to actuate the die set, such that the required crimping force is directly related to the hydraulic pressure within the cylinder. FIGS. 2 through 4 illustrate three commonly used approaches to the application of pressure during a crimp cycle. FIG. 2 shows a crimp cycle in which the pressure gradually and nonlinearly increases during the cycle. FIG. 3 shows an example of the pressure being increased and then held steady to finish the crimp. In FIG. 4, the pressure increases but is then allowed to drop toward the end of the cycle. Various other pressure cycles are also possible, and the following discussion is not limited to the pressure cycles represented in FIGS. 2 through 4.
Typical crimping equipment and dies are designed to accommodate a range of hose and fitting sizes and types. It can be appreciated that the crimping force required to produce a reliable crimp increases as the diameters of the fittings increase. It is also true that heavier hoses and fittings, in other words, those that contain more material that must be compressed during crimping, also require much greater crimping forces than lighter hoses and fittings of the same diameter. This relationship is represented in FIG. 5, and can be seen to be nonlinear.
Another consideration during crimping is how the hose 12 and fitting 14 respond to a crimp. Due to material being compressed and plastically deformed during crimping, the diameter of the fitting 14 (as measured by the diameter of the ferrule 20 in FIG. 1) naturally expands slightly after a crimp toward its original shape, a phenomenon that will be referred to as fitting spring-back. The amount of spring-back is a significant part of the crimping process that varies from fitting to fitting. FIG. 6 represents that heavier fittings result is more spring-back than lighter ones, again in a nonlinear relationship.
In addition to the above factors, the pressure necessary to attain a required crimp diameter is further complicated by tolerances and variations in manufacturing hoses and fittings. Tolerances may result in different crimping forces/pressures being required to attain a required crimp diameter for a quantity of a particular type of hose/fitting combination.
In an effort to more reliably attain a desired crimp diameter, crimpers typically utilize either pressure or position of the hydraulic cylinder to adjust the crimp diameter. FIG. 7 shows a flow chart for a crimper controlled by pressure. Monitoring pressure is usually only effective for very light and small fittings, especially those manufactured to relative tight tolerances so that the pressure required to achieve a desired crimp diameter is more consistent from crimp to crimp. As the fittings and hoses get bigger, the size variation between fittings tends to increase due to tolerances, with the result that the pressure to achieve a desired crimp diameter can significantly vary from fitting to fitting, for the reasons discussed previously with respect to FIG. 5. For crimp cycles that operate with a gradually increasing pressure as represented in FIG. 2, monitoring the pressure to control the crimp can generally be reasonably achieved. However, for crimp cycles of the type shown in FIGS. 3 and 4, reliable control of the pressure to obtain a desired crimp diameter becomes complicated. As a result, monitoring pressure to control a crimping cycle is very limited and not widely used.
In view of the above, positional feedback is more widely used as a method for making crimps. This type of control system is represented in FIG. 8. To illustrate the following discussion, reference is made to FIGS. 9 and 10, which show a crimper 22 whose die set 24 is in fully open (expanded) and fully closed (contracted) positions, respectively. The die set 24 is shown as being actuated with a piston 26 of a hydraulic cylinder 27. Typically, the linear travel of the piston 26 can be directly correlated to the diameter Dd of the die set 22 through knowledge of the camming surfaces 32 between the die set 24 and piston 26. As such, the position of the piston 26 relative to a suitable reference is often monitored with this control method. A limit switch or position transducer (not shown) is typically mounted externally on the crimper 22 to provide feedback of the position of the piston 26. The limit switch or transducer must be mounted on the crimper 22 away from the hose and fitting (not shown) so that it is not damaged during use of the crimper 22 and does not interfere with the crimping process. For this reason, for crimpers similar to the type shown in FIGS. 9 and 10, the position of the piston 26 is usually monitored based on the distance between the piston 26 and the front plate 28 of the crimper 22.
As seen in FIG. 10, the front plate 28 tends to deflect during the crimp cycle as a result of the die set 24 being retained in the crimper 22 with the front plate 28. This deflection affects any position measurements of the piston 26 taken with respect to the front plate 28. As fittings vary, the required pressure varies, and the deflection in the front plate 28 also varies, with higher pressures resulting in greater deflection as shown in FIG. 11. FIG. 12 shows another relationship that exists between the die set diameter Dd and the deflection of the crimper front plate 28. From FIG. 12, it can be understood that a position reading made during crimping of a fitting that causes only minor deflection will result in a smaller die set diameter Dd than the same position reading for a different fitting whose crimping causes significant deflection. As such, monitoring crimper position generally works well as a control method, but can be inaccurate due to crimper deflection.
In view of the above discussion, current crimper technology requires the operator to constantly monitor the crimping process and make adjustments as required. Though a given hose and fitting combination will have unique specifications that require certain crimper settings to achieve a desired crimp diameter Dc, the operator must also adjust for variances in individual hoses and fittings attributable to tolerances. Many crimper settings do not provide for any spring-back in a hose and fitting assembly, necessitating that the operator measure each crimped assembly and re-crimp to a smaller diameter if the assembly is out of tolerance. Crimp settings also do not allow for crimper deflection, representing yet another reason for operators to measure each crimped assembly and re-crimp if necessary. Because the crimp diameter will typically be out of tolerance due to a combination of fitting spring-back and crimper deflection, it is not uncommon for some hose/fitting assemblies to require multiple crimps before their crimp diameter falls within an acceptable range.
From the above, it can be appreciated that, while crimp operators can identify and fix defective crimps as they occur, it would be desirable if variables that cause defective crimps could be automatically compensated for.